A recent trend in the audio industry is the use of matrix surround encoded stereo, for example Dolby Surround encoding, in audio signals. These audio signals may accompany analog or digital video signals, which together form a television signal, VCR signal, or CD or DVD signal. In Dolby systems, the encoder combines four different signals, i.e. left, right, center, and surround and produces two output signals, i.e. left total and right total. A further discussion of Dolby encoding, Surround decoders, and Pro Logic decoders may be found in “Dolby Pro Logic Surround Decoder Principles or Operation” by Roger Dressler, which is hereby incorporated by reference. The main difference between matrix surround decoders, for example a Dolby Pro Logic decoder and other decoders, is the way steering and relative balance between output channels is achieved.
The Surround decoder receives the two output signals, left total and right total, from the encoders and produces three output signals, i.e. left, right, and surround. The common or mono information contained in the left and right channels of a matrix surround encoded recording carries the center channel information, and thus the center signal is reproduced as a phantom image between the left and right speakers. The Surround decoder is typically a passive decoder as it uses (L−R) difference amplifier. The Pro Logic decoder receives the two output signals, left total and right total from the encoder, and produces four output signals, i.e. left, right, center, and surround. However, the Pro Logic decoder uses an adaptive matrix to continuously monitor the encoded audio signals, evaluate the inherent sound field dominance, and apply processing in the same direction and proportion to that dominance.
A problem with existing systems is that a listener has to use more than two speakers to get the desired surround effect from a sound source. This not only costs more in terms of the number of speakers to be purchased to get the desired sound effect, but also requires a room of adequate size so that the different speakers producing the left, right, center and surround sound may be placed at sufficient distances from the listener and also from each other to produce the desired sound effects. Thus, listeners with limited resources are not able to afford the multiple speaker systems or are unable to find means for placing the speakers to achieve the desired sound effects.
Another problem occurs when Dolby encoded audio materials are decoded on QSound systems. QSound systems use “Q-filters” in the processing of audio signals. The Q-filters could be part of a “QXpander circuit,” wherein QXpander is a registered trademark of QSound. Also, the term “QXpander circuit” is a descriptive term used for the purpose of this application to refer to the filters more specifically described in U.S. Pat. No. 5,440,638 to Lowe et al., which is hereby incorporated by reference. The Q-filters could be Q1 filters. The term “Q1 filter” is also a descriptive term used for the purpose of this application to refer to the filters more specifically described in U.S. Pat. Nos. 5,105,462 and 5,208,860 both to Lowe et al., wherein each of these patents are hereby incorporated by reference. A Q-filter adjusts the amplitude and phase of an input signal on a frequency dependent basis. Note that the Q-filters use phase inverted signals during processing to achieve the QSound virtual audio image effects. Consequently, if an input signal to a Q-filter is already inverted from Dolby encoding, then the Q-filter system will re-invert the input signal and then proceed with processing of an improper, re-inverted signal. A re-inverted or non-inverted signal will adversely affect the expansion mechanisms of the Q-filter. Thus, the output signal will result in the incorrect placement of sound images. In other words, the surround images would appear to be located at the left and right speakers, and not placed to the sides or rear of the listener. Note that these effects would occur on other expansion mechanisms that use phase inversion.
Therefore, there is a need in the art for a mechanism which will correctly apply expansion mechanisms to both non-inverted signals and inverted signals.